[Boatanchors] SB-220 with parasitics

[JAMES D COMSTOCK]

I would guess the "parasitic bang" would have been a flashover. This is usually caused by gas in the tube. The gas can come from overheating the elements which boils the gas from the metal or more commonly with a 3-500, a tiny leak of air into the tube. The flashover does clear some of the gas buildup. It may or may not have damaged the tube beyond use. The balance of the gas buildup can be cleared by gettering. The getter is on the plate of that tube and is not active until the plate is beginning to glow red hot. Gettering without causing a flashover is done by maintaining high current at low plate voltages for some time. But if flashovers continue to happen intermittently  after it has been gettered then there is likely air slowly entering the tube. That was a common problem with EIMAC tubes with latter build dates. Jim



> On 12/24/2021 9:23 AM Keith Densmore <ve3gem at gmail.com> wrote:
> 
>  
> Safe Holidays to Everyone,
> I have agreed to look at a Heath SB-220 amp for a friend. The amp, tubes
> look like new but one of the 3-500's has had a parasitic bang, at least
> once and maybe twice. (On the same tube.)
> I cannot find much info on the web but the question is, can such a tube be
> reformed? Run the filament for some hours? Power it up with lower plate
> and/or filament voltage? Any hope or is it dead?
> If anyone has experience in this area, your advice is welcome.
> 
> Thanks,
> Keith ve3gem
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